Rainbow Electronics MAX1817 - Manual (Page 8)

MAX1817
the output voltage rises above the undervoltage thresh-
old, the control circuitry is enabled, allowing proper
regulation of the output voltage.
LCD Step-Up Converter
The MAX1817’s LCD step-up converter generates an
LCD bias voltage up to 28V by use of a 500mA, 1.1
Ω
internal N-channel switching MOSFET (Figure 1). The
LCD step-up converter control circuitry is powered from
the main step-up converter output (OUT), so the voltage
at OUT must be above the OUT undervoltage lockout
voltage for the LCD step-up converter to operate.
During startup, the MAX1817 extends the minimum off-
time to 5µs for V
FBLCD voltages <0.9V, limiting initial
surge current. The LCD step-up converter features an
independent shutdown control, ONLCD.
The LCD step-up converter features a minimum-off-
time, current-limited control scheme. A pair of one-
shots that set a minimum off-time and a maximum on-
time governs the duty cycle. The switching frequency
can be up to 500kHz and depends upon the load, and
input and output voltages.
Compact, High-Efficiency, Dual-Output
Step-Up DC-DC Converter
8 _______________________________________________________________________________________
ON
FB
V IN
ON
ON
OFF
OFF
ONLCD
SHUTDOWN
LOGIC
MAIN
UNDERVOLTAGE
LOCKOUT
DUAL-MODE FEEDBACK
BIAS
LCD
STARTUP
ERROR
COMPARATOR
MAIN
CURRENT
LIMIT
MAIN
ZERO-
CROSSING
DETECTOR
CONTROL
LOGIC
MAIN
1.25V
AGND
AGND
GND
OUT
LX
FBLCD
LXLCD LCD
MAIN
GND
GND
SHUTDOWN
LOGIC
LCD
ERROR
COMPARATOR
LCD
1.25V
AGND CURRENT
LIMIT
LCD
CONTROL
LOGIC
LCD
MAX1817
75mV
Figure 1. MAX1817 Simplified Functional Diagram